Conventionally, a metallic medical device which is used in general remains in a body unless the medical device is removed by an operation or the like after being embedded in the body. Depending on applications, it is desirable that such a metallic medical device maintains a mechanical strength during a period that peripheral tissues are recovered, and is degraded and disappears without requiring an operation after the recovery of the peripheral tissues.
A magnesium material shows high corrosion rate in an environment where chloride ions are present and hence, the use of the magnesium material as a general-purpose-use material such as parts of transportation equipment or household appliances is limited. On the other hand, the magnesium material shows low harmful property to a living body, and is corroded, is degraded and disappears at an extremely high speed in an approximately neutral aqueous solution containing chloride ions such as a body fluid. Accordingly, the magnesium material is expected to be used as a medical biodegradation metallic material which is gradually degraded and absorbed after being embedded in a living body, so that the development of the magnesium material has been under way (see patent documents 1 and 2).
Depending on a kind of a device or a condition of an affected part, a strength holding period which the device is required to satisfy varies in an extremely wide range. For example, with respect to a blood vessel treatment device such as a stent, it is desirable that the device maintains a strength for a period of five months to six months necessary for repairing a narrowed part of a blood vessel, and the degradation of the whole device is almost finished within one week to twelve weeks after the vessel is repaired. This is because when the stent remains in the blood vessel even after the blood vessel wall is repaired, due o the mechanical stimulation and chemical irritation which the stent continuously contacts to the blood vessel wall, vascular endothelial cells are excessively grown thus causing restenosis of the blood vessel whereby the disappearance of the stent after repairing of the blood vessel is extremely important.
On the other hand, with respect to a fracture fixation device, it is desirable that the device supports a load during three months to one year until fracture is cured and, thereafter, the degradation of the whole device is almost finished within eight month to five years.
In this manner, along with the degradation and disappearance of the device after the fracture is cured, a load is gradually applied to a cured bone and hence, it is possible to suppress the load interception which is a phenomenon that the device supports the load in place of the bone. This leads o the suppression of re-fracture which occurs due to bone absorption (thinning of bone) attributed to the load interception. Further, it is unnecessary to perform an operation for taking out the device after the fracture is cured and hence, a burden imposed on a patient can be reduced. In this manner, the strength holding period which the device is required to satisfy varies in a wide range, and may be a long period of several months or more in some cases.
Accordingly, it is considered desirable if it could be possible to control the progress of degradation during a period in which the strength holding is required and a following period in which degradation progresses. However, in case of the biodegradable magnesium material proposed in patent document 1, for example, the degradation period is controlled corresponding to a size of device. Accordingly, in a living body where the degradation of the device starts immediately after embedding of the magnesium material and, at the same time, a space in which the device is to be embedded is limited, there may be a case that the device having a necessary size is not applicable whereby, it is substantially impossible to properly use the magnesium material as a device which is required to hold a strength for a particularly long period.
Further, with respect to a biodegradable magnesium material which inventors of the present invention proposed in patent document 2, the magnesium material is configured to control a strength-ductility balance of the material and a degradation speed of the material in a living body to desired values based on the composition of the material or a control of the internal structure of the material. For example, the degradation speed can be controlled by controlling a kind and the concentration of an adding element.
However, the control of the degradation speed due to the formation of an alloy using an adding element is limited and hence, the adjustment of the degradation period in a wide range is difficult. That is, the concentration of the adding element in the alloy is dependent on and defined by a desired strength-ductility balance and hence, the limitation of the adjustment range of corrosion resistance is inevitable.    Patent document 1: JP-A-2004-160236    Patent document 2: International Publication WO2007/58276 brochure